1. Field of the Invention
The present invention relates to a receiver for RF signals, and more particularly to a receiver for frequency-modulated signals.
2. Description of the Prior Art
An FM-receiver is known, e.g., "radio mentor electronic" 32 (1966) No. 6, in which a very low intermediate frequency of about 120 to 180 kHz is generated from the received signal by means of a mixer and a local oscillator; this low-frequency IF signal is selected with RC filters and amplified and then applied to a counting discriminator for FM detection. This known principle suffers from the disadvantage that the signals of an adjacent FM transmitter appear as image frequencies if the oscillator frequency lies between these two transmitter frequencies. The principle has another disadvantage in that, during tuning, each transmitter appears twice, i.e., if the oscillator frequency lies about 150 kHz above and below the transmitter center frequency.
Quite independently of FM receiver engineering, a system of single sideband amplitude modulation is known, "Funkschau" 1972, No. 14, p. 485, which is referred to in the literature as the "third method." In that method, for receiving the sideband, the local oscillator is not tuned to the transmitter's carrier frequency but to the center of the single sideband to be received. The received signal and the oscillator signal are fed to two separate mixers, with either the received signal or the oscillator signal being fed to one mixer with a 90.degree. phase shift in relation to the other mixer. The low-frequency signals obtained in this way by down - conversion are separated, with one lowpass filter each, from adjacent frequencies, e.g., also from the adjacent sideband not to be received, and the carrier is then added again in a low-frequency modulator. These two signals are thus shifted in the low-frequency range in such a manner that, through the additive mixing, the audio signal demodulated by multiplicative mixing in the low-frequency modulator appears with a superposed inverted audio signal. These two quadrature signals are fed to an adding circuit where the audio signals add together, while the inverted audio signals substract and cancel. This "third method," which is used with amplitude modulation, places extremely stringent requirements on the accuracy of the 90.degree. phase shifts and on the symmetry of the addition. Errors of as little as 1% result in the signal-to-noise ratio being reduced to quite a disturbing value (40 dB signal-to-noise ratio). This interference is due to the fact that the signals appearing behind the first mixers and selected in the low-pass filter are frequency-shifted audio signals and that separation from the interfering inverted signal is possible only by means of a bridge circuit (addition or subtraction). In an improvement of this "third method," transmitters with carrier not suppressed are detected by replacing each of the last low-frequency modulators with a full-wave rectifier.
For the reception of telegraph transmitters using frequency-shift keying it is also known to split the output voltages behind the second mixers once again into a direct signal and a signal in phase quadrature and to add the quadrature signal of one mixer to that signal of the other mixer which has not been shifted in phase. This gives two complementary digital signals which are alternately in the 0 or 1 state when the transmitter frequency is above or below its center frequency. This principle is applicable exclusively to digital signals, i.e., telegraph signals, because the criterion evaluated is only whether the received frequency lies above or below a center frequency to which the receiver is tuned. It cannot be used to form intermediate values as are necessary for the transmission of analog signals.